1. Field of the Invention
The present invention relates to apparatus and methods for monitoring the pressure within a cavity in a patient, and more specifically, but not by way of limitation, to devices and methods for telemetrically monitoring the intracranial pressure of a patient.
2. Description of Related Art
The need for monitoring intracranial pressure in patients has long been recognized. Elevated intracranial pressure may be tolerated for only a few hours or perhaps as long as days or weeks. In all circumstances, unmonitored and uncontrolled elevated intracranial pressure will eventually lead to cerebral injury and can lead to visual loss or to cerebral white matter injury and dementia.
The general pathophysiological process that can elevate intracranial pressure regardless of the specific disease includes brain tumors, pseudotumor cerebri, hydrocephalus, severe head trauma and other situations where patients are subject to brain swelling, edema, obstruction of cerebral spinal fluid pathways or intracranial space occupying lesions. Convenient and accurate monitoring of the intracranial pressure in these situations frequently allows correctional emergency procedures when intracranial pressure rises or falls to dangerous levels.
Currently available methods for monitoring intracranial pressure involve measuring cerebral spinal fluid pressure via a lumbar puncture as well as more directly measuring pressure using a catheter passing through the skull and the scalp to some data acquisition system exterior to the patient. In some cases, the catheter is simply a plastic tube which vents the subarachnoid pressure to an electronic readout pressure gauge. A more recently developed class of sensors involve a microminiature pressure transducer located in the distal end of the catheter. The catheter is passed through the brain tissue placing the pressure transducer into one of the ventricles. A fiberoptic filament communicates the measured pressure data to the external data acquisition portion of the system.
Such catheter type systems are generally unsatisfactory for long term monitoring of intracranial pressure because of the inherent dangers of: infection; patient discomfort; reduced patient mobility; and surgical procedures necessary to remove the pressure transducer when it is no longer needed.
Long term intracranial pressure monitoring without undue risk of these dangers requires a totally implantable sensor that dan survive structurally and provide a measurable and meaningful response to intracranial pressure for extended periods of time. In the 1970's and early 1980's, the Applied Physics Laboratory of John Hopkins University (JHU/APL), under the direction of Applicant, designed and developed a telemetric intracranial pressure monitoring system. The telemetric intracranial pressure monitoring system developed by JHU/APL comprised a sensor which was implanted into a burr hole in the skull wall; and a telemetric data acquisition system which eliminated the need for a wire or catheter coupling as the method of communication between the sensor and the data acquisition system.
The sensor that was developed by JHU/APL is disclosed in U.S. Pat. No. 4,265,252, issued to Chubbuck et al. on May 5, 1981. The sensor comprises a passive device consisting of a RF resonant circuit having a resonant frequency varying from about 50 MHz to about 100 MHz.
The telemetric data acquisition system developed by JHU/APL is disclosed in U.S. Pat. No. 4,114,606, issued to Seylar on Sep. 19, 1978. The Seylar patent discloses a data acquisition system comprising a high frequency sweeping signal generator with crystal markers. The sweeping signal generator would drive an interrogation coil through a 50 ohm coaxial cable. The interrogation coil was placed externally of a patient's skull but close to the sensor so that a signal showing the frequency at which the sensor resonated was displayed on an X-Y display oscilloscope. Medical personnel would then convert the signal displayed on the X-Y display oscilloscope to the actual intracranial pressure by way of referring to calibration charts and barometric correction charts.
The telemetric intracranial pressure monitoring system developed by JHU/APL was developed and tested from about 1976 through about 1983. During this time, 127 implantable sensors were implanted in human patients. The results of these studies were reported in journals from about 1979 through about 1988 and typical articles are "Continuous Intracranial Pressure Recording in Adult Hydrocephalus", written by G. Gucer, L. J. Viernstein and A. E. Walker, Surg. Neurol. 13:5, May 1980; and "Ten-Year Follow-up on the Performance of a Telemetric Intracranial Pressure Sensor", written by G. Gucer, L. J. Viernstein, A. Wang and R. Szymanski, Neurosurgery 22:5 1988.
The results of these studies verified that the sensor of the telemetric intracranial pressure monitoring system reliably produced an intracranial pressure measurement. However, the zero pressure resonant frequency of the sensor (or calibrated baseline) changed or drifted not due to actual intracranial pressure changes. More specifically, the zero pressure reading of the implantable sensor increased 1.0.+-.0.2 mm H.sub.2 O/day (0.074 mm Hg/day) over a period of 6.8.+-.1.6 years.
Furthermore, it was found that a subgroup of implantable sensors displayed a decreasing zero pressure reading not due to actual intracranial pressure decreases. The decreasing zero pressure reading was found to be caused by seepage of body fluids into the implantable sensor.
It was reported during the study that only some of the nurses involved in the study were able to make the conversion from the resonant frequency of the implantable sensor to the intracranial pressure of the patient. It was ultimately concluded that: the bulk of the laboratory test equipment used to read the implantable sensor was inconsistent with space allocation requirements of intensive care units; the technical expertise required to operate the test equipment was not likely to be found among intensive care personnel; and reference to calibration and correction charts complicated the data acquisition system's use by medical personnel.
The increasing and decreasing of the sensor's zero pressure reading is commonly referred to in the art as "baseline drift" because the zero pressure reading of the implantable sensor drifts away from a calibration baseline not due to actual intracranial pressure changes.
Based on these experiences, the systems disclosed in the Chubbuck and Seylar patents were not as suitable as is desired to be utilized in the hospital environment.
Thus, a need exists for an improved telemetric intracranial pressure monitoring system for the long term continuous or intermittent monitoring of a patient's intracranial pressure. However, such an improved telemetric intracranial pressure monitoring system must also be cost efficient and substantially maintenance-free. It is to such an improved telemetric intracranial pressure monitoring system that the present invention is directed.